Boiler economizer and control system

Abstract

An increased efficiency boiler is provided, which, instead of trying to eliminate steaming in the economizer, designs the economizer to permit steaming, and a control is provided for the boiler which takes into account the heat input to the boiler as well as the water level in the steam drum and the reliability of the economizer.

What is claimed is: 
     
       1. In a natural circulation boiler, comprising an evaporators, a drum connected to the evaporator, and an economizer upstream of and in fluid communication with the evaporator; said economizer including a plurality of substantially vertical multiple pass fluid flow modules in fluid communication with each other; each multiple pass module including a bottom header, a top header, and a plurality of tubes extending between and in fluid communication with said bottom header and said top header, and at least one baffle in one of said headers, so that fluid entering the multiple pass module must flow at least once upwardly from the bottom header to the top header and at least once downwardly from the top header to the bottom header before leaving the multiple pass module, the improvement comprising: at least one upwardly-flowing single pass module at the downstream end of said multiple pass fluid flow modules, said upwardly-flowing single pass module including a top header and a bottom header and a plurality of tubes extending between and in fluid communication with said respective top and bottom headers, wherein fluid enters said single pass module at the bottom header, travels only up to the top header and then out of the module, said single pass module having no downwardly-directed portion, so that, if steaming occurs at the end of the economizer, it will not be trapped in a downwardly-directed tube.   
     
     
       2. In a boiler as recited in claim 1, wherein there are two paths which the fluid leaving the economizer can take--a path into the evaporator and a bypass path in which the fluid leaving the economizer does not go into the evaporator; and further comprising a bypass valve in the bypass path, controlling the amount of fluid which takes the bypass path. 
     
     
       3. In a boiler, comprising an evaporator including a plurality of evaporator tubes, a drum for separating water and steam; a downcomer which directs water from the drum into the evaporator tubes and a collection header which receives the output from a plurality of the evaporator tubes and directs the output from said evaporator tubes into said drum; and an economizer including an output conduit; the improvement comprising: the output conduit from said economizer is in fluid communication with said collection header, so that fluid leaving said economizer can enter said drum through said collection header, thereby avoiding the need for a special feedwater pipe from the economizer to the drum.